The present invention relates to a system for controlling an electrostatic precipitator by using digital signal processing and, in particular, to the control of the precipitator in response to at least its secondary voltage and secondary current in an individual half cycle of an alternating power source.
Known control means for detecting precipitator operating conditions employ averaging techniques whereby the precipitator power is cycled, i.e., increased and reduced, during many half cycles of an alternating power source to ascertain a characteristic voltage-current response for the precipitator secondary. When the characteristic response is ascertained, the precipitator power is adjusted to provide optimal precipitation. However, since the characteristic response is always changing based on varying operating conditions in the precipitator, periodically the cycling must be repeated looking for a change in the characteristic response. If a change has occurred, the power must be adjusted in response thereto.
A disadvantage of the averaging technique discussed above is that it requires numerous half cycles to ascertain the characteristic voltage-current response for the precipitator. Since it is a goal of precipitation to operate the precipitator at a power level as high as possible without causing strong back corona or excessive sparking, the averaging technique leads to inefficiency because the precipitator may be required to spend numerous half cycles operating under inefficient conditions.
For example, a back corona condition occurs in a precipitator when particulate matter or dust forms on at least one plate of the electrostatic precipitator such that a continuous breakdown of the dust layer occurs. This breakdown is analogous to that occurring at the discharge electrode and similarly produces ion-electron pairs. The positive ions flow across the interelectrode region toward the discharge electrode. The net effect is a reduction of charge on the particles and poor precipitation. During such a condition, the current being supplied to the precipitator plate becomes consumed in the back corona instead of being used to precipitate the suspended gas particles.
The prior art technique for responding to the undesirable back corona condition is to employ the averaging technique described above to detect the point at which voltage no longer increases while current continues to increase and to reduce the current sufficiently so as to operate the precipitator at or below this point. By reducing the current sufficiently, the back corona condition is minimized so that power flowing to the precipitator is used for precipitating particulate matter rather than to feed the back corona. However, since an averaging technique is employed, the system is required to spend numerous half cycles operating in the inefficient back corona area.
Another example of the inefficiencies associated with the prior art techniques is in the control of precipitator rapping. Rapping is generally used to remove collected dust or particulate matter from the precipitator plates. As dust increases on precipitator plates, the resistivity of the plates, including the dust layer, also increases. This increase in resistivity can occur rapidly and increases the probability of sparking. In known control means, rapping is usually a function of time, gas flow or opacity, but not the electrical conditions in the precipitator such as resistivity. As a result, known control means are not able to rapidly identify and respond to fast changing resistivity conditions in the precipitator.
Accordingly, there is the need for a precipitator control system that responds dynamically to precipitator operating conditions, such as back corona, during each half cycle of an alternating power source and that is capable of adjusting the precipitator drive and other precipitator operating parameters in response to such conditions following each half cycle.